Simultaneous determination of the cosmic ray ionization rate and fractional ionization in dr 21(OH)

We present a new method for the simultaneous calculation of the cosmic ray ionization rate, zeta H-2, and the ionization fraction, chi(e), in dense molecular clouds. A simple network of chemical reactions dominant in the creation and destruction of HCNH+ and HCO+ is used in conjunction with observed pairs of rotational transitions of several molecular species in order to determine the electron abundance and the H-3(+) abundance. The cosmic ray ionization rate is then calculated by taking advantage of the fact that, in dark clouds, it governs the rate of creation of H-3(+). We apply this technique to the case of the star-forming region DR 21( OH), where we successfully detected the (J = 3 -> 2) and (J = 4 -> 3) rotational transitions of HCNH+. We also determine the C and O isotopic ratios in this source to be C-12/C-13 = 63 +/- 4 and O-16/O-18 = 318 +/- 64, which are in good agreement with previous measurements in other clouds. The significance of our method lies in the ability to determine N(H-3(+)) and chi(e) directly from observations, and estimate zeta H-2 accordingly. Our results, zeta H-2 = 3.1 x 10(-18) s(-1) and chi(e) = 3.2 x 10(-8), are consistent with recent determinations in other objects.